In order to form Large scale integration circuit (LSIs), and thin-film transistors (TFTs) for liquid crystal displays on glass substrates, there has been technology well established in which a photoresist is used to form the necessary device structure or circuit structure pattern that is in turn etched in an etching step for removal of unnecessary portions thereby obtaining the desired device structure or circuit structure.
The device structure or circuit structure to be formed may have a variety of configurations depending on the desired electronic device. In recent years, however, a variety of structures using silicon nitride film materials have also been under study. Typically in the process for producing non-volatile memory devices like flash memory devices, a gate insulating film pattern and a gate electrode pattern are formed on a semiconductor substrate. These electrode structures generally comprise an electrode material that is made conductive by incorporating a dopant in polysilicon and an insulating film formed of SiO2, SiN or SiON. When the electrode structure further includes a floating gate and a control gate, there is an insulating layer formed of ONO (oxide-nitride-oxide), SiON or SiN so as to insulate them. To this end there is a step provided in which the semiconductor substrate is etched across the gate electrode pattern to form a trench.
For a DRAM memory cell structure too, the structure of a silicon nitride film is adopted. Typically in a stack type memory cell, a device isolation area is formed on a P-type silicon substrate to form a gate oxide film and a gate electrode. Then, a source/drain area is formed, and an insulating film is further formed by CVD or the like, with a contact provided on it. Then, there is a polycrystalline silicon film formed for electric conductivity, which film is in turn patterned to form a lower electrode of a capacitor. Then, a thin silicon nitride film is formed by CVD all over the surface to provide the capacitor with an upper electrode. Then, a BPSG film (insulating film) is formed by CVD, and a contact for connection to a bit line for data retrieval is opened using known photolithographic/etching technology. Finally, an electric conductive film and a BPSG (insulating film) are formed to obtain a stack type DRAM memory cell.
The silicon nitride film is also used for the formation of a photodiode proximity structure of a conventional CCD imaging device including a device isolation mechanism having an STI structure. Specifically, a silicon substrate is provided on it with an insulating film comprising a silicon oxide film, and the insulating film is provided on it with a protective film comprising a silicon nitride film or the like. This protective film is provided on it with a resist pattern having an opening or aperture in the area having the device isolation trench formed in it. With this resist pattern as a mask, the insulating film and protective film are etched to form an opening through which the upper surface of the silicon substrate is exposed. Note here that the resist pattern will be removed by asking or the like after the formation of the opening. Then, the protecting film is used as a mask for etching of the silicon substrate thereby forming a device isolation trench.
Further, the silicon nitride film structure is also adopted for a pixel-driving thin-film transistor (TFT) or the like in a display device such as a liquid crystal device. For instance, the ohmic contact layer of a channel portion of the thin-film transistor is dry etched, after which a passivation layer in a silicon nitride film form is locally provided.
Thus, the silicon nitride film is increasingly used for various electronic devices. One of the reasons is the fact that such silicon nitride films differ from mask materials composed mainly of silicon such as silicon oxide and oxygen in terms of anticorrosion capability. This capability makes sure a silicon nitride layer is used as a stopper layer for dry etching, a corrosion preventive layer, a mask or the like in the production of semiconductor devices. The silicon nitride film may be used in combination with a low-dielectric-constant insulating material of the silicon oxide type represented by Low-k material or the like and containing oxygen and other elements such as fluorine and carbon for patterning or acquiring selective etching capability when they are used as a mask.
However, a problem with such etching or cleaning steps is that when the etching solution and cleaning solution used corrode the aforesaid compounds, the silicon film containing nitrogen is also more or less corroded. In particular, recent microstructures of devices are likely to be subject to noticeable adverse influences even from a slight corrosion; so there is mounting demand for the development of more advanced corrosion preventive technology.
Translation of PCT Application No. 2004-528716 goes deep into adverse influences of etch selectivity on such materials as silicon oxides in the step of wet etching materials containing silicon and nitrogen such as silicon nitride and silicon oxynitride. However, what is considered there is a problem with the etching of such a material as silicon oxide at the time of etching a material containing silicon and nitrogen, and that publication says or suggests nothing about the reverse phenomenon whatsoever. In addition, the aforesaid problem is only solved by performing etching in a dilute aqueous solution containing hydrofluoric acid (HF) in a concentration range of 0.001M to 0.1M at a temperature of 25° C. to 90° C. In other words, the temperature and concentration of the aqueous solution of hydrofluoric acid are nothing else than defined. For this reason, any study of prevention of corrosions by the introduction of other compound elements is not made at all.